. Alona falcata — Alonella rostrata. 305 



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Alona falcata, Sars. 



Plate XIII, Figs. 9, 10. 



This species was found not very rare in dredgings from shal- 

 low water in Washington Harbor, Isle Royale and in lake 

 Gogebic, Michigan. There are some minute differences between 

 my specimens and some of the European descriptions. Sars' 

 account ('62, p. 162,) is more like our form than are those of 

 P. E. Mueller ('68, p. 183,) or Hellich ('77, p. 95). Both of 

 "these authors are less accurate in their account of the posterior 

 margin of the valves than is the original description of Sars. 

 There is a distinct dorso-posteal angle, and the margin is 

 sinuate. The infero-posteal angle is rounded and the teeth, 

 which number 1-3, are set, not on the margin but a little 

 within it. The cilia of the ventral margin are long, especially 

 at the anterior and posterior ends. 



The post-abdomen is well described by Sars as "crassum fere 

 teres. " The dorsal lower margin is carried almost parallel to 

 the ventral margin of the valves and the organ is used in loco- 

 motion somewhat as it is employed by Dunhevedia. 



Alonella rostrata, Koch. 



1878, Birge, Pleuroxus acutirostris] p. 23, pi. II, f. 15. 



This species was found at Minocqua, and more abundantly 

 from Isle Royale and also in collections from Easthampton, 

 Mass. It has not been recognized before in this county al- 

 though I earlier described it as a new speciec of Pleuroxus. Its 

 affinities are more nearly with Pleuroxus than with Alona. The 

 single spine of the postabdomen is the chief character in com- 

 mon with Alo?ia, while shell markings and beak are Pleuroxine. 

 I see no ground for including the forms with short beaks — 

 P. excisus and exiCjuus — in the same genus with this distinctly 

 long-beaked form. 



